As described in my U.S. patent application Ser. No. 12/543,342, herein incorporated by reference in its entirety, a perfect bound book comprises a book block having a plurality of text pages, one edge of which constitutes a spine. The book typically has a one piece cover of a stock heavier than the stock from which the text pages are printed. The cover has a center spine area that is adhesively bound to the spine of the book by a suitable adhesive (preferably a hot melt adhesive) that has been applied to the spine at an adhesive application station immediately prior to being brought into contact with the cover. As shown in FIGS. 6 and 9 of the above-identified U.S. patent application, the adhesive application station typically includes a heated reservoir in which a quantity of the desired hot melt adhesive is melted and contained. The adhesive is maintained at a suitable elevated temperature (e.g., up to about 400° F., depending on the adhesive). The adhesive application station has a rotary wheel or drum that is partially immersed in the liquid adhesive such that when the wheel is rotated about a horizontal axis, the periphery of the wheel will be coated with the adhesive such that when the spine of the book block is brought into engagement with the wheel and when the wheel is moved lengthwise along the spine (or when the spine is moved relative to the wheel), the adhesive will be transferred to the spine.
Because of this, the low density molecules in the adhesive are evaporated (boiled off) thus altering the chemistry and viscosity of the hot melt adhesive. This, in turn, changes the application and performance of the bind.
It has also been found that the viscosity of the adhesive is inversely proportional to the temperature of the mixture and inversely proportional to the amount of low density molecules in the mixture. Viscosity of the adhesive has an important bearing on the speed and height of the glue wave (as described in my above-noted U.S. patent application) to achieve a proper application of glue to the spine of the book block. As the level of low density molecules are reduced, it becomes more difficult to “wet” the spine of the book block because the more viscous adhesive has more surface tension, which inhibits the application of the adhesive to the spine. Of course, if the spine is not properly wetted by the liquid adhesive, the adhesion of the adhesive to the spine and the adhesive of the cover to the adhesive on the spine is diminished. This adversely affects the strength of the bind. Thus, replacement of the supply of adhesive in the heated reservoir required. Of course, when it becomes necessary to drain the old adhesive from the reservoir, to replace the adhesive with new adhesive, and to bring the new adhesive up to operating temperature, this requires the POD book publishing system to be out of service for up to about an hour.
It will be understood that for print on demand (POD) book publishing systems, such as disclosed in the above-identified U.S. patent application, books are not continuously produced, but rather are typically produced as they are ordered by customers. Accordingly, significant periods of time may pass between the time one book is produced and the next book has been ordered. Because it takes time to heat the adhesive in the reservoir (e.g., 30-45 minutes or more), the reservoir is typically maintained in its heated state such that a book may be immediately produced upon the system being commanded to print the next book. As further disclosed in the above-noted U.S. patent application, one preferred adhesive is adhesive HM8101 commercially available from Capital Adhesives of Mooresville, Ind. However, other hot melt adhesives may be used, depending on a number of factors. Typically, such hot melt adhesives are complex organic compounds and admixtures having a wide array of molecules of different molecular weights, and different temperature and vaporization characteristics. As these hot melt adhesives are maintained at their elevated application temperatures in the heated reservoir for extended periods of time, certain of the more volatile organic compounds will vaporize and outgas from the adhesive thus changing the makeup of the remaining adhesive. If sufficient amounts of the more volatile compounds are vaporized, the adhesive or binding characteristics of the remaining adhesive may be adversely affected, thus resulting in improper binding of the book. Thus, from time-to-time, it has been necessary to change the adhesive in the reservoir to insure that satisfactory binding will result.
However, it has been difficult to judge when the adhesive in the reservoir should be replaced with fresh adhesive. It will be understood that upon changing the adhesive, it may again take a considerable time to allow the new adhesive to be heated to its desired application temperature (e.g., about 30-45 minutes, or more), during which time the apparatus will be down unable to bind books. It will be appreciated that if the print on demand (POD) book publishing system is used to produce books in sufficient quantity, adhesive must be added to the reservoir on a regular basis such that the adhesive is not resident in the reservoir for extended periods and such that the adhesive characteristics of the adhesive may be maintained for extending periods of time such that it is not necessary to drain and replace the adhesive in the reservoir On the other hand, if the POD system is only used occasionally and if the adhesive remains heated in the reservoir for extended periods so that the POD system will be ready for use with little or no warm-up period, the quality of the adhesive in the reservoir may be degraded relative quickly, depending on the characteristics of the adhesive being used. Thus, it is only desirable to change the adhesive when the adhesive in the reservoir has degraded to the point where it will not satisfactorily bind books. However, there has been a long-standing problem for the operator of the POD system to know when it would be desirable to change the adhesive in the reservoir so as to maintain the quality of the bound books without having to unnecessarily change the adhesive, which will result in the POD apparatus being out of service.